OK @Words, one large element of the A level spec, as already mentioned is analysis of a book and a film (or two books) which calls for skills beyond basic knowledge of language.
And as I have stated, the rest of the course is looking at political, social and cultural themes that also demand a fair amount of research and extra learning. So this will need to be done by the student, native speaker or not.
What the merit is of this - well it’s certainly true that some candidates take an A level in their native language to get an "easy A/A-star" - I've come across that. I still think it has some merit, as unless they do some work (which is where the value comes in for me) they won't get an A, as I have mentioned.
And as for the qu of their scores not being counter-weighted - while I agree that this presents an issue, and the greater the proportion of native speakers the worse that is, I genuinely don't see what you can do. There is such a range of "native speaker". My mother spoke another language than English until she was five - so that was her mother tongue - my name could suggest I spoke it too - but do I? no, not a word.
Someone could have a German name and speak no German; or be called John Smith and be fluent. The only way to assess would be to ask candidates to self-declare (and then mark them down in some way, or make the GB higher) and I don't see how that could be policed. I mean who will check that they are telling the truth?
I think the idea of redressing the balance at university (in terms of grade required, or support once there) is a good one and more workable.
If a non-native speaker works hard and puts in the time and effort, and has a good level of skill and knowledge from GCSE to build on, and is able enough at languages, they can gain an A-star. Native speakers don't make that impossible. Candidates who reach a certain standard will get the top mark - and that does not vary radically from year to year.